Arc tube for a discharge lamp

Abstract

An arc tube for a discharge lamp has a closed chamber filled with rare gas and a metal halide containing at least Na halide or Sc halide, and electrodes, wherein sealing pressure of the rare gas is 0.6 MPa or more, and a sealing density of the Sc halide in the closed chamber is ranging from 1.25 to 4.70 mg/ml. In the arc tube, since the Xe gas sealing pressure slightly higher than 0.6 MPa, the pressure in the closed chamber is increased during its lightening state. Thus, reactions leading to a flicker occurrence may be accelerated. However, the flicker occurrence can be suppressed by setting the Sc halide sealing density to 4.70 mg/ml or less. Further, a luminous efficiency required for the lamp can be assured by setting the Sc halide sealing density to 1.25 mg/ml or more.

1. An arc tube for a discharge lamp, comprising:
 a closed chamber filled with rare gas and a metal halide containing at least Na halide and Sc halide, an internal volume of the closed chamber being 50 μl or less; and 
 electrodes provided so as to be opposed to each other, 
 wherein sealing pressure of the rare gas is 0.6 MPa or more, 
 a sealing density of the Sc halide in the closed chamber is in the range of 2.00 to 4.70 mg/ml; 
 wherein the closed chamber is made of SiO 2 ; and 
 wherein Zn halide is also included in the closed chamber. 
 
   
   
     2. An arc tube for a discharge lamp as set forth in  claim 1 , wherein the metal halide further containing In halide. 
   
   
     3. An arc tube for a discharge lamp as set forth in  claim 1 , wherein the metal halide further contains Sn halide. 
   
   
     4. An arc tube for a discharge lamp as set forth in  claim 1 , wherein the Sc halide is ScI 3 . 
   
   
     5. An arc tube for a discharge lamp as set forth in  claim 1 , wherein the closed chamber is made of Al 2 O 3 . 
   
   
     6. An arc tube for a discharge lamp as set forth in  claim 1 , wherein the electrode is made of W. 
   
   
     7. An arc tube for a discharge lamp as set forth in  claim 6 , wherein the electrode contains ThO 2 . 
   
   
     8. An arc tube for a discharge lamp as set forth in  claim 1 , wherein the rare gas is a Xe gas. 
   
   
     9. An arc tube for a discharge lamp as set forth in  claim 1 , wherein a halogen in the metal halide is I. 
   
   
     10. An arc tube for a discharge lamp, comprising:
 a closed chamber filled with rare gas, mercury and a metal halide containing at least Na halide and Sc halide, an internal volume of the closed chamber being 50 μl or less; and 
 electrodes provided so as to be opposed to each other, 
 wherein sealing pressure of the rare gas is 0.6 MPa or more, and 
 a sealing density of the Sc halide in the closed chamber is in the range of 1.25 to 4.70 mg/ml. 
 
   
   
     11. An arc tube for a discharge lamp as set forth in  claim 10 , wherein the sealing density of the Sc halide in the closed chamber is in the range of 2.00 to 4.70 mg/ml. 
   
   
     12. An arc tube for a discharge lamp as set forth in  claim 11 ,
 wherein arc tube is a quartz-glass arc tube; 
 wherein the metal halide includes NaI and ScI 3 ; 
 wherein the rare gas is Xe; 
 wherein the electrodes are formed of straight electrode rods made of thoria-doped tungsten; and 
 wherein there is a mechanical gap of 3.8 mm between the electrodes.